The invention pertains to power electronics and, particularly, to extending the operating range of metal oxide semiconductor field effect transistors (MOSFETs) used in cryogenic applications.
Metal oxide semiconductor field effect transistors (MOSFETs) are used as switches in electronic circuits. MOSFETs, which can be turned "on" to pass electrical currents and "off" to stop those currents, are controlled electrically and operate extremely rapidly. Thus, they are ideal for use in high-speed switching applications, such as power electronics, where current flows are sometimes started and stopped millions of times per second.
As with most electrical devices, MOSFETs are constrained to operate within specific power, temperature and switching ranges. For example, a MOSFET that is operating at room temperature, with a gate-to-source voltage of 20 volts, and a switching rate of 10,000 cycles/sec, may be limited to passing currents on the order of 10 amps. Though the so-called safe operating areas (SOAs) of MOSFETs are well known, it is often difficult to insure that the SOA of a given MOSFET will not be exceeded, at least momentarily, during circuit operation. This is particularly true where a MOSFET is operated close to the limits of its SOA.
Transients are a well known phenomena in electrical circuits. When MOSFET devices are switched on and off, they induce transients in the surrounding circuitry. Those transients may prove harmless for MOSFETs operating well within their SOAs. However, as the operational limits of the MOSFETs are approached, the transients may push voltages and currents beyond the SOAs, causing electrical ruptures of the delicate metallic and semiconductor structures within the devices.
Although the prior art suggests the use of "snubber" circuitry to squelch the deleterious effect of transient on MOSFETs, that additional circuitry can be costly when used in the quantities required for power electronics, where banks of hundreds of MOSFETs may be employed in switching large currents.
In view of the foregoing, an object of the invention is to provide improved power electronics and, particularly, improved MOSFET switching circuitry.
A more particular object of the invention is to provide power MOSFET switching circuitry for use in cryogenic applications, such as low temperature superconducting (LTS) devices and high temperature superconducting (HTS) devices.
A still further object of the invention is to provide such power MOSFET switching devices that can operate in cryogenic environments close to the limits of their SOAs without risk of failure.
Yet still another object of the invention is to provide such power MOSFET switching devices as can be implemented at low cost.
A still further object of the invention is to provide such power MOSFET switching devices as occupy a minimum of space.